Currently available are various types of ion sources adapted for the generation of ion beams including closed electron drift (or closed Hall current) ion sources. Such ion sources are subdivided into two types: ion sources with an extended acceleration zone comprising a dielectric channel (for example, European application EP 0 541 309 A1, IPC H05H1/54, F03H 1/100, published May 12, 1993), and ion sources with a short acceleration zone (for example, the patent U.S. Pat. No. 4,122,347, IPC H01J 27/00, published Sep. 24, 1978), which are also referred to as anode layer ion sources. The second type of closed electron drift ion sources is most generally employed for processing purposes, because:                a separate electron emitter is not needed for operation of such ion sources,        such ion sources are free from ion acceleration dielectric channels,        the anode-to-cathode distance may be minimal,        anode layer ion sources are of simpler design and provide for generation of extended cylindroid ion beams of large linear sizes,        the ion source enclosure is generally used as a cathode.        
The Russian patent RU 2030807 (IPC H01J 27/04, 37/08, published Mar. 10, 1995) describes a closed electron drift ion source designed for the generation of extended cylindroid beams. The prior art ion sources comprise an enclosure fabricated from magnetically permeable material and adapted for serving as a cathode. A gas distributor communicated with the ion source enclosure is designed for supplying a working gas into a discharge gap. An elongated emission hole made in the discharge end wall of the ion source enclosure is defined by parallel rectilinear portions and two closing curved portions. An anode is symmetrically positioned in the cavity of the ion source enclosure, opposite to the slot-shaped emission hole. The anode is positioned axisymmetrically around permanent magnets which are located between the ion source enclosure end walls to generate a magnetic field in the working gap of the slot-shaped emission hole. Parts of the magnetically permeable discharge end wall of the ion source enclosure opposite to the slot-shaped emission hole are acting as pole pieces of the magnetic system of the device. The pole pieces together with magnetically permeable components of the enclosure are electrically isolated one from another and are grounded through current measuring devices. The configuration and intensity of the generated ion beams is controlled by changing the profile of the emission hole and the pole piece distance. On the whole, the design of the prior art ion source provides for reduced sputtering of the pole pieces and, as a result, an increased purity of the ion beam, reduced contamination in sputtered depositions and improved quality of the ion beam processed articles surfaces.
There exists a number of constructive designs directed to improve the magnetic system for closed electron drift ion sources resulting in increased intensity of the ion beam. As an example, a prior art closed electron drift ion source comprises an improved magnetic system (the patent U.S. Pat. No. 5,763,989, IPC H 05 H1/02, published Jun. 9, 1998). The magnetic system for such an ion source comprises magnetomotive force sources made in the form of permanent magnets, a magnetic circuit and magnetically permeable shields surrounding the coaxial discharge channel of the device. The magnetic system is adapted for generation in the discharge channel of a radial magnetic field with predetermined axial field gradient. This ion source belongs to the class of plasma devices with an extended acceleration zone and needs an additional electron emitter positioned behind the coaxial discharge channel section.
The closest prior art embodiment of the present invention is an extended cylindroid ion beam source based on the principle of acceleration of ions in an anode layer with closed electron drift (the patent U.S. Pat. No. 4,277,304, IPC H01L 21/306, H01 J 17/04, published Jul. 7, 1981). In this embodiment, the ion source enclosure is provided with a closed loop slot-shaped exit hole for ion emission and for generation of an extended cylindroid ion beam. The anode of the ion source is positioned inside the enclosure cavity opposite to the exit hole. A working gas distributor is communicated with the cavity of the enclosure and, accordingly, with the ion source discharge channel. A cathode of the prior art ion source is the enclosure or a portion of the enclosure and the end wall with the closed loop exit hole for ion emission. The ion source end wall with the exit hole is fabricated from magnetically permeable material (magnetically permeable steel).
The prior art technical design is directed to the creation of a compact extended cylindroid beam ion source and to the reduction of leakage of magnetic flux by the optimized construction of the magnetic system. The magnetic system for the ion source comprises permanent magnets arranged on outside of the enclosure, along edges of the closed loop slot-shaped exit hole. The magnetic field induction vectors of permanent magnets arranged at opposite edges of the exit hole are oriented parallel to the direction of ion emission and have opposite polarity. The enclosure end wall with an elongated closed loop exit hole is fabricated from magnetically permeable material. Parts of the enclosure end wall separated by the closed loop exit hole serve as pole pieces of the magnetic system and define a magnetic working gap along the closed loop exit slot.
In the prior art, pole pieces are also positioned on outer end surface parts of permanent magnets and adapted for conducting the magnetic flux around the outside of the enclosure thereby preventing the magnetic flux from leaking outside the discharge channel. In some embodiments of the device (see, as an example, FIG. 6 of the patent U.S. Pat. No. 4,277,304), outer pole pieces of the magnetic system are connected by means of magnetic flux conducting jumpers. Thus, the outside pole pieces of the prior art ion source are designed only for concentrating the magnetic flux within the cavity of the magnetic circuit enclosure and do not serve as magnetic elements defining an additional magnetic working gap. Acceleration of ions in such device is effectuated in crossed electric and magnetic fields in the region of the magnetic working gap adjacent to the anode. The pole pieces arranged on end surfaces of permanent magnets and interconnected through magnetic flux conducting jumpers, as is shown in FIG. 6 of the patent
U.S. Pat. No. 4,277,304, define the exit hole of the ion source and do not exert a substantial effect upon the ion beam formation process. In another specific embodiment of the ion source (see FIG. 8A of the patent U.S. Pat. No. 4,277,304), the pole pieces positioned on end surfaces of permanent magnets define a second magnetic gap, where additional acceleration of ions in crossed electric and magnetic fields is theoretically possible. However, the patent U.S. Pat. No. 4,277,304 does not indicate specific conditions determining the distribution of the magnetic field in the magnetic lens, which forms at the exit hole of the ion source and serves to generate an ion beam. Hence, it is unjustified to draw the conclusion of a possible effect of the second magnetic gap of the magnetic lens used in the prior art device upon the ionization and the ion acceleration processes.
Among significant operating parameters for ion sources used in processing are ion beam current density, uniformity of ion current density across the ion beam section and the electric discharge stability. The above characteristics, in their turn, depend upon the precise dimensions of the emission hole, uniform distribution of magnetic and electric fields in the magnetic working gap, as well as upon uniformity of a working gas supply along the emission hole. The fulfillment of these conditions is of particular value for generation of extended cylindroid beams. In addition, these conditions promote obtaining the desired high ion current density per unit of length of the emission hole by enabling the production of high-power density discharges in the working gap behind the emission hole. On the whole, currently available ion sources are not suited to producing the above mentioned required conditions and, because of this, have limited possibility for application in a broad range of desired ion beam processes.